1. Field of the Invention
The present invention relates to an electrolytic processing apparatus and an electrolytic processing method, and more particularly to an electrolytic processing apparatus and an electrolytic processing method useful for processing a conductive material formed in a surface of a substrate, such as a semiconductor wafer, or for removing impurities adhering to a surface of a substrate.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistivity and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in a surface of a substrate. Various techniques for forming such copper interconnects are known including chemical vapor deposition (CVD), sputtering, and plating. According to any such technique, a copper film is formed in a substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 1A through 1C illustrate, in sequence of process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 1A, an insulating film 2, such as an oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer 1a in which semiconductor devices are formed, which is formed on a semiconductor base 1. Contact holes 3 and interconnect trenches 4 are formed in the insulating film 2 by a lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the insulating film 2, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5 by sputtering or CVD, or the like.
Then, as shown in FIG. 1B, copper plating is performed on the surface of the substrate W to fill the contact holes 3 and the interconnect trenches 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6, the seed layer 7 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) or the like so as to make the surface of the copper film 6 filled in the contact holes 3 and the interconnect trenches 4, and the surface of the insulating film 2 lie substantially on the same plane. Interconnects composed of the copper film 6 as shown in FIG. 1C are thus formed.
Components in various types of equipments have recently become finer and have required higher accuracy. As sub-micro manufacturing technology has commonly been used, the properties of materials are largely influenced by the processing method. Under these circumstances, in a conventional machining method in which a desired portion of a workpiece may be physically destroyed and removed from the surface by a tool, a large number of defects may be produced which deteriorates the properties of the workpiece. Therefore, it becomes important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with the conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects such as formation of an altered layer and dislocation due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
An electrolytic processing method that utilizes an ion exchanger has been developed. As shown in FIG. 2, an ion exchanger 12a mounted on a processing electrode 14 and an ion exchanger 12b mounted on a feeding electrode 16 are allowed to be close to or into contact with the surface of a workpiece 10. A voltage is applied from a power source 17 to between the processing electrode 14 and the feeding electrode 16 while a processing liquid 18, such as ultrapure water, is supplied from a fluid supply section 19 to between the processing electrode 14, feeding electrode 16 and the workpiece 10, thereby carrying out removal processing of the surface layer of the workpiece 10. According to this electrolytic processing, water molecules 20 in the processing liquid 18, such as ultrapure water, are dissociated by the ion exchangers 12a and 12b into hydroxide ions 22 and hydrogen ions 24. The hydroxide ions 22 thus produced, for example, are carried, by the electric field between the workpiece 10 and the processing electrode 14 and by the flow of the processing liquid 18, such as ultrapure water, to the surface of the workpiece 10 facing the processing electrode 14, whereby the density of the hydroxide ions 22 in the vicinity of the workpiece 10 is increased, and the hydroxide ions 22 are reacted with the atoms 10a of the workpiece 10. The reaction product 26 produced by reaction is dissolved in the processing liquid 18 such as ultrapure water, and removed from the workpiece 10 by the flow of the processing liquid 18 along the surface of the workpiece 10.
When carrying out electrolytic processing of e.g. copper by using as an ion exchanger a cation exchanger having a cation-exchange group, copper is captured by the cation-exchange group. Progress of the consumption of the cation-exchange group by copper makes it impossible to continue the electrolytic processing. When electrolytic processing of copper is carried out by using as an ion exchanger an anion exchanger having an anion-exchange group, on the other hand, fine particles of a copper oxide are produced and the particles adhere to the surface of the ion exchanger (anion exchanger), whereby particles may harm the uniformity of the processing rate.
Such harmful effects can be eliminated by regenerating the ion exchanger. Regeneration of an ion exchanger is effected by exchange of an ion captured by the ion exchanger for a hydrogen ion in the case of a cation exchanger, and for a hydroxide ion in the case of an anion exchanger.
Regeneration of an ion exchanger is generally carried out by immersing the ion exchanger in an acid solution in the case of a cation exchanger, and in an alkali solution in the case of an anion exchanger. With a cation exchanger which has captured an ion having an ion selectivity coefficient close to that of a hydrogen ion, such as a sodium ion, for example, the ion exchanger can be regenerated in a very short time by immersing it in an acid. When an ion exchanger which has captured an ion having a large ion selectivity coefficient is regenerated with an acid solution or an alkali solution, however, the generation speed is very slow. Further, a chemical remains in a high concentration in the regenerated ion exchanger, which necessitates cleaning of the ion exchanger. Further, it is necessary to separately provide a regeneration tank for storing a regeneration liquid which may occupy a considerable installation area. In addition, processing must be stopped for regeneration of an ion exchanger, leading to lowering of the throughput.
An ion exchanger to be in contact with a workpiece, from the viewpoint of a smooth surface, is in the form of, for example, a thin film. Accordingly, the ion-exchange capacity, which is an index of the ion accumulation capacity, is generally small. It is, therefore, a common practice to interpose an ion exchanger having a large ion-exchange capacity between an ion exchange in a film form and an electrode so that most of processing products may be taken in the interposed portion (interposed ion exchanger). After carrying out processing for some length of time, however, the interposed portion does not take processing products anymore, and therefore must be changed for a new one or regenerated. The change of the ion exchanger takes a considerable time during which processing must be stopped. The regeneration also necessitates a stop of processing, which adversely affects the throughput of the apparatus.
In view of this, it may be considered to discharge metal ions, etc., which have been removed from a workpiece and taken in an ion exchanger, to a discharge section (out of the system) during processing by using, for example, an electrodialysis regeneration method, thereby regenerating the ion exchanger during processing. In such an in-process regeneration method, however, metal ions, etc., which have newly dissolved out of the workpiece, are constantly taken into the ion exchanger during regeneration. Thus, depending upon the conditions, all the metal ions taken into the ion exchanger could not be fully discharged out of the system.